When operating at high frequencies, normal track conductors are replaced with strip lines, in order to ensure the necessary isolation. Such a strip line includes at least one strip-shaped signal conductor and at least one ground plane, with the signal conductor running at a distance to the ground plane. So-called symmetric strip lines in which the signal conductor is arranged between two ground planes are also possible. The strip lines are advantageously arranged in multiple-layer substrates that each have several dielectric layers with metallization planes arranged in between. Ground planes and signal conductors are each realized in different metallization planes arranged one above the other.
The impedance of each strip line is determined by a capacitive and an inductive transmission-line constant. The impedance of a strip line is essentially a function of the width of the strip-shaped signal conductor and the distance of the signal conductor from the closest ground plane, in addition to the permittivity of the substrate material. Lowering the capacitance and thus increasing the impedance is achieved when the width of the strip-shaped signal conductor is reduced and/or the distance of the signal conductor from the closest ground plane is increased.
In practice, the distance between the signal conductor and the closest ground plane is limited by the thickness of the multiple-layer substrate in which the strip line is realized, wherein, for a strip line constructed as a symmetric strip line, half the thickness of the substrate is available as the maximum signal conductor/ground plane distance. Also, the width of the signal conductor is limited by technology, because, for example, in ceramic multiple-layer components, the conductor width cannot be reduced arbitrarily.
If long strip lines or signal conductors are provided in a multiple-layer substrate, then the signal conductor/ground plane also determining the impedance of the strip line cannot be increased arbitrarily for substrate thicknesses that are common today for highly miniaturized components and modules. This has the consequence that the strip line in the substrate often cannot reach a desired standard impedance of, for example, 50 Ω. This problem is made worse if symmetric strip lines are used exclusively in the substrate because of better isolation.